In a communication network system such as the UMTS terrestrial radio access network (UTRAN), there are two potential bottlenecks, namely, the air interface and the transport network (TN or Iub) (transport link) connecting the radio network controller (RNC) and NodeB. The transport link between the RNC and NodeB is a potential bottleneck when its capacity is smaller then the available maximal capacity of the UMTS Air Interface (Uu). For example, a typical scenario is that the NodeB is connected to the RNC through an E1 link with a capacity of approximately 2 Mbps, and in this case the available Uu capacity for the high speed downlink packet access (HSDPA) may be significantly larger than 2 Mbps. This means that a single user equipment (UE) with good radio conditions can overload the transport network (TN).
The fair sharing of Uu resources is the task of the Uu scheduler, but the Uu scheduler can not cope with the TN bottleneck, i.e. the transport link bottleneck. In order to deal with the TN bottleneck a flow-control (FC) mechanism has been introduced. The goal of the FC is to efficiently use the TN through ensuring that the TN resources are used well while keeping low frame loss and low delay in the TN.
Lack of FC causes serious performance degradation when the transport network or the air interface is the bottleneck. In this case the TN buffer is typically full, causing high TN delay and loss ratio. This causes exhaustive radio link control (RLC) retransmissions which results in a much lower throughput.
In current HSDPA systems both uplink (UL) and downlink (DL) can provide higher throughput compared to ancestor systems, like GSM, and EDGE. Features including shared channel transmission, high order modulation, fast radio link adaptation, and channel dependency scheduling etc can guarantee higher data rate and system throughput in the DL air interface.
However, in some scenarios there may be an unbalance between the transport interface and the air interface. For example, the transport network interface is limiting if the transport interface is only 1 or 2 E1/TI based, and, the air interface may also be limiting if the transport interface is a very good wired based connection. The even more likely scenario is when the air interface limitation and transport interface limitation are mixed. The air interface throughput is fluctuating because the radio quality depends on the user equipment (UE) mobility. Hence, it is very necessary to have flow control (FC).
FC located in NodeB interacts through Iub Frame Protocol messages (capability allocation (CA) control frame messages) with the MAC-d function in the RNC. A well designed FC algorithm will guarantee the Iub resources to be used efficiently while keeping a low frame loss and delay over the Iub interface. To guarantee an efficient utilization of the air resources, the FC entity ensures that the priority queue (PQ) (i.e. the scheduler buffer length for a user UE in the NodeB) is short enough when Uu is bad, and ensures that the PQ is long enough when Uu is good. Often only non-guaranteed bit rate (non-GBR) flows, i.e. flows that can suffer packet loss and delay under congestion, are flow controlled.
The HSDPA FC entity, located in NodeB, makes sure that there is a good balance between Iub and Uu interface, and exchanges the CA control frames between the NodeB and SRNC. High resource utilization should be guaranteed no matter either side interface is limited.
A state of the art FC algorithm adjusts the CA per MAC-d PDU flow. As already mentioned the FC ensures the PQ, i.e. the scheduler buffer length for a UE in the NodeB, to be short enough with small CA when Uu is bad, and ensures the PQ to be long enough with large CA when Uu is good. When the Uu is bad a handover may be evident which would empty the PQ during handover, which causes the data that are not transmitted yet, to be lost, so that it has to be retransmitted by higher layer protocols, e.g. RLC. When the Uu is good the data rate over the Uu may be so high that the PQ can not offer enough bits to be transmitted over the Uu and thus the user throughput is lower than it should as the user data bits resides too long in the RLC SDU buffer. Hence, the FC is highly important for the system performance.
A simplified illustration of flow control as applied in a communication network system such as the UMTS terrestrial radio access network (UTRAN), is illustrated by FIG. 6. In order to achieve an optimal use of a scheduler buffer PQ in NodeB for a given user equipment UEi communicating with NodeB, CA for user i is sent to the RNC via the Iub interface, transport network, requesting the amount of data to be sent in a later predefined period. The requested data for user i is then sent from the RNC data buffer, via the Iub interface, transport network, to the NodeB PQ. The NodeB PQ provides data to the given user equipment UEi communicating with said NodeB as an over-air transmission using the Uu interface, air interface.
PQT (the time to serve PQ by NodeB scheduler) may be one input to indicate Uu congestion. The Data Frame loss or Delay over Iub are used to measure TN congestion. The PQT is estimated via an algorithm. However, sometimes the PQT is overestimated, which result in that the PQ runs empty, i.e. not enough bits are requested from the RLC SDU buffer, thus the CA is to low. But, sometimes the PQT is underestimated, CA too large, which may result in an increased RLC retransmission rate or temporary RLC window stalling.
FIG. 3 and FIG. 4 demonstrates the comparison results between a max Channel Quality Indicator (maxCQI) scheduler and Round Robin (RR) scheduler in terms of PQT deviation and the PQ nonempty ratio.
The positive value of PQT deviation means that the PQ is overestimated and the negative value of the PQT deviation means that the PQ is underestimated. From the C.D.F. graph in FIG. 3, it can be seen that the maxCQI scheduler has higher PQ estimation variation, and also higher probability to overestimate the PQ than the RR scheduler, i.e. a higher probability of the PQ running empty. An overestimate means that the PQ contains less data than it can transmit or maybe even is empty whereas an underestimate means that the PQ contains more data than it can transmit.
FIG. 4 shows the comparison of PQ non-empty ratio of transmission time intervals (TTIs). The maxCQI scheduler has a higher probability of running empty than the RR scheduler. One of the reasons is that RR is fairer in the time domain than maxCQI. The PQT estimation algorithm is mainly based on metrics in the time domain.
FIGS. 3 and 4 clearly illustrates that different HSDPA schedulers have different FC performance. RR has least probability of empty PQ while worse in terms of cell throughput compared to maxCQI and proportional fairness (PF) schedulers. PF and maxCQI schedulers have a higher probability to overestimate the PQ. RR schedulers have a lower probability to run the PQ empty than maxPQI and PF schedulers. If the PQ runs empty the cell throughput will be decreased.